This invention relates to a method and apparatus for providing correction signals to the beam of a cathode ray tube in proportion to the proximity of the beam relative to those preselected locations where correction factors had been previously recorded.
In a typical cathode ray tube (CRT) display system, correction signals are required if the size and shape of the beam is to remain constant as the beam is moved on the face of the CRT, independent of the location of the beam. This is particularly important in high resolution systems. Among the correction factors normally applied are focus, X and Y position corrections, astigmatism, and shading.
Traditionally, analog waveform generation is used in a horizontal (X) and vertical (Y) raster CRT display to provide spot size correction such as dynamic focus (a function that lies between X.sup.2 +Y.sup.2 and the square root of (X.sup.2 +Y.sup.2) or to provide geometrical correction such as X.sup.2 Y and Y.sup.2 X for pincushion correction, etc. For example, one such system is described in an article entitled "Dual Mode Calligraphic/Raster Color Cathode Ray Tube (CRT) Projector" by Richard E. Holmes in Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol. 386, January, 1983.
These corrections are mathematically derived approximations of the desired correction waveforms. In reality higher order terms than typically used are needed, but they are not practical to implement. Higher resolution monochrome displays may need dynamic astigmatism correction as well as focus and geometry correction. Color displays need dynamic convergence correction such as red, green, blue and blue lateral correction. Basically, though, the waveform generator can be considered as a device which generates a unique Z (magnitude) output for every X and Y location on the display.
Attempts have been made to use digital correction in the past. The article entitled "Twenty-Five-Inch Precision Color Display for Simulator Visual Systems" by R. E. Holmes and J. A. Mays in Optical Engineer, Vol. 18, No. 6, November-December 1979, pp. 630-633, describes a technique of combining analog circuitry with two dimensional digital correction using a checker board approach where the value for the vertical (Y) direction remained the same for several scan lines. Smoothing between digital points was achieved in the X direction by means of analog filtering which kept the number of values saved in digital memory for this dimension within practical limits. In the vertical direction, the concept became impractical because so many scan lines of values are needed that memory size becomes excessive. If too few scan lines of vertical correction data are used, a basketweave pattern will appear on the screen due to discontinuity that appears at each boundary of the checker board pattern. What is needed is a practical means to provide vertical (Y) smoothing, and the present invention fulfills that need.
In one embodiment of the present invention, two sets of electronically programmable read-only memories (EPROMs) are used to store a 40.times.80 element digital representation of the correction waveforms which would be suitable for a 1280.times.1280 picture element video display. Smoothing for the 40 element representation of the 1280 horizontal components is still done by analog means, mainly by taking advantage of the inductances of the deflection, focus, astigmatism and convergence coils of magnetically focused and deflected displays. For electrostatic displays, resistive, inductive, capacitive or active filtering, well known in the art, can be used to perform this function.
The data in the two sets of EPROMs are identical but shifted by one block. For example, one EPROM holds correction data for lines 0, 16, 32, . . . , 1248 and 1264 while the other EPROM holds correction data for lines 16, 32, 48, . . . , 1264 and 1280. The same functions might be implemented with only one EPROM if the data from it is digitally time multiplexed into two separate latches.
For one sequence of 16 scan lines, the first EPROM generates 40 different digital values representing the actual values that should exist along the top scan line. Simultaneously, the other EPROM generates 40 different digital values representing the actual value that should exist along the scan line n+16. For equivalent X direction points along Y direction lines n+1, n+2, . . . n+16, a good estimate value would be the linear weighted average. For instance, the smoothed value for line n+4 would be (n+4) where: EQU (n+4)=[(n+16)-(n)]4/16+(n).
The system is not limited to a raster scan type system. It can also be used with point writing CRT systems where the address for the EPROMs is derived from analog-to-digital (A/D) converters sampling the sweep signals or from those signals used to direct the beam.
In the preferred embodiment of the invention, the digital output from each EPROM is connected to a video digital-to-analog converter, and the outputs from these devices are connected to a resistance ladder across which a fast analog multiplexer selects an interpolated value in accordance with the line being scanned. As an alternative, very fast and not inexpensive digital circuits could be used in place of the resistance ladder and analog multiplexer circuit.
Although the invention is described in connection with a cathode ray tube display system, the technique described is equally usable for any N dimensional waveform generation where linear interpolation is desired for intermediate values of digitally generated numbers.
Accordingly, it is an object of this invention to provide a method in connection with a CRT display system where correction factors for the beam have been recorded in memory for a few preselected positions on the face of the CRT comprising the steps of selecting the location where the beam of the CRT is to be placed, selecting those memory locations containing correction factors pertaining to the selected beam location, determining the relative contribution of the selected memory locations for the selected beam location, and combining the correction factors in accordance with their relative contribution.
It is also an object of this invention to provide an apparatus for providing correction signals to the beam of a CRT in accordance with the location of the beam on the screen, said apparatus comprising memory means, said memory means including memory locations corresponding to a limited number of preselected locations on said screen where correction factors to the characteristics of the beam have been recorded, means for selecting the location where the beam is to be placed, means for selecting those memory locations closest to the beam, and means for combining the correction factors recorded in said selected memory locations in proportion to the proximity of the beam relative to the preselected locations where the correction factors were recorded in said memory means.
It is also an object of this invention to provide a correction circuit that may be used with a point-to-point cathode ray tube display system.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.